calendarswikiaorg-20200214-history
Second
The second (symbol: s''') is the of in the (SI) |accessdate=2012-03-24}} and is also a unit of time in other (abbreviated '''s or sec ). Between 1000 (when used seconds) and 1960 the second was defined as 1/86,400 of a mean (that definition still applies in some astronomical and legal contexts).International System of Units from NIST accessed 25 March 2012. Between 1960 and 1967, it was defined in terms of the period of the Earth's orbit around the Sun in 1900, | url=http://tycho.usno.navy.mil/leapsec.html | accessdate=2012-03-24}} but it is now defined more precisely in atomic terms. Seconds may be measured using mechanical, electric or atomic s. 19th- and 20th-century astronomical observations revealed that the mean solar day is slowly but measurably lengthening and the length of a is not entirely predictable either; thus the sun–earth motion is no longer considered a suitable basis for definition. With the advent of s, it became feasible to define the second based on fundamental properties of nature. Since 1967, the second has been defined to be: periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.}} es are frequently combined with the word second to denote subdivisions of the second, e.g., the (one thousandth of a second), the (one millionth of a second), and the (one billionth of a second). Though SI prefixes may also be used to form multiples of the second such as (one thousand seconds), such units are rarely used in practice. The more common larger non-SI units of time are not formed by powers of ten; instead, the second is multiplied by 60 to form a minute, which is multiplied by 60 to form an hour, which is multiplied by 24 to form a . The second is also the base unit of time in the , , , and systems of units. International second Under the International System of Units (via the , or CIPM), since 1967 the second has been defined as the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the 133 atom. In 1997 CIPM added that the periods would be defined for a caesium atom at rest, and approaching the theoretical temperature of , and in 1999, it included corrections from ambient radiation. This definition refers to a atom at rest at a temperature of 0 ( ). Absolute zero implies no movement, and therefore zero external radiation effects (i.e., zero local and s). The second thus defined is consistent with the , which was based on astronomical measurements. (See below.) The realization of the standard second is described briefly in a special publication from the , and in detail by the . |accessdate=2009-08-19 }} Equivalence to other units of time 1 international second is equal to: * 1/60 (but see also ) * 1/3,600 * 1/86,400 ( system of units) * 1/31,557,600 (IAU system of units) History Before mechanical clocks The Egyptians subdivided daytime and nighttime into twelve hours each since at least 2000 BC, hence the seasonal variation of their hours. The astronomers (c. 150 BC) and (c. AD 150) subdivided the day ly and also used a mean hour day)}}, simple fractions of an hour ( , , etc.) and time-degrees ( day or four modern minutes), but not modern minutes or }} The day was subdivided sexagesimally, that is by , by of that, by of that, etc., to at least six places after the sexagesimal point (a precision of better than 2 microseconds) by the ns after 300 BC. For example, six fractional sexagesimal places of a day was used in their specification of the length of the year, although they were unable to measure such a small fraction of a day in real time. As another example, they specified that the mean synodic month was 29;31,50,8,20 days (four fractional sexagesimal positions), which was repeated by Hipparchus and Ptolemy sexagesimally, and is currently the mean synodic month of the , though restated as 29 days 12 hours 793 (where 1 hour = 1080 halakim). |isbn=0-387-06995-X }} The Babylonians did not use the hour, but did use a double-hour lasting 120 modern minutes, a time-degree lasting four modern minutes, and a barleycorn lasting 3 modern seconds (the of the modern Hebrew calendar),See page 325 in |volume=22 |pages=321–360 |doi= |id= }} but did not sexagesimally subdivide these smaller units of time. No sexagesimal unit of the day was ever used as an independent unit of time. In 1000, the scholar gave the times of the new moons of specific weeks as a number of days, hours, minutes, seconds, thirds, and fourths after noon Sunday. In 1267, the medieval scientist stated the times of full moons as a number of hours, minutes, seconds, thirds, and fourths (horae, minuta, secunda, tertia, and quarta) after noon on specified calendar dates. |page=table facing page 231 |isbn=978-1-85506-856-8 |nopp=true |others=BR Belle }} Although a third for of a second remains in some languages, for example (tercja) and (salise), the modern second is subdivided decimally. Seconds measured by mechanical clocks The earliest clocks to display seconds appeared during the last half of the 16th century. The earliest spring-driven timepiece with a second hand which marked seconds is an unsigned clock depicting in the Fremersdorf collection, dated between 1560 and | location=Cambridge, Massachusetts| publisher= Harvard University Press |year= 1983 | isbn = 0-674-76802-7 }} full page color photo: 4th caption page, 3rd photo thereafter (neither pages nor photos are numbered).}} During the 3rd quarter of the 16th century, built a clock with marks every 1/5 minute.Taqi al-Din In 1579, built a clock for that marked }} In 1581, redesigned clocks that displayed minutes at his observatory so they also displayed seconds. However, they were not yet accurate enough for seconds. In 1587, Tycho complained that his four clocks disagreed by plus or minus four }} The second first became accurately measurable with the development of s keeping mean time (as opposed to the apparent time displayed by sundials). In 1644, calculated that a pendulum with a length of 39.1 inches (0.994 m) would have a period at one of precisely two seconds, one second for a swing forward and one second for the return swing, enabling such a pendulum to tick in precise seconds. In 1670, clockmaker added this to the original pendulum clock of . |volume=1 |issue=0 |pages=1 |doi=}} From 1670 to 1680, Clement made many improvements to his clock and introduced the longcase or to the public. This clock used an mechanism with a seconds pendulum to display seconds in a small subdial. This mechanism required less power, caused less friction and was accurate enough to measure seconds reliably as one-sixtieth of a minute than the older . Within a few years, most British precision clockmakers were producing longcase clocks and other clockmakers soon followed. Thus the second could now be reliably measured. Modern measurements As a unit of time, the second (meaning the second division by 60 of an hour) entered English in the late 16th century, about a hundred years before it was measured accurately. Those who wrote in Latin, including scientists like , and , used the Latin term secunda with the same meaning as far back as the 1200s. In 1832, proposed using the second as the base unit of time in his millimeter-milligram-second . The (BAAS) in 1862 stated that "All men of science are agreed to use the second of mean solar time as the unit of time." BAAS formally proposed the in 1874, although this system was gradually replaced over the next 70 years by units. Both the CGS and MKS systems used the same second as their base unit of time. MKS was adopted internationally during the 1940s, defining the second as 1/86,400th of a mean solar day. In 1956, the second was redefined in terms of a year (the period of the 's revolution around the Sun) for a particular because, by then, it had become recognized that the Earth's rotation on its own axis was not sufficiently uniform as a standard of time. The Earth's motion was described in (1895), which provided a formula for estimating the motion of the Sun relative to the epoch 1900 based on astronomical observations made between 1750 and 1892. | url=http://tycho.usno.navy.mil/leapsec.html | accessdate=2006-12-31 }} The second was thus defined as: This definition was ratified by the Eleventh in 1960, which also established the . The in the 1960 definition was not measured but calculated from a formula describing a mean tropical year that decreased linearly over time, hence the curious reference to a specific instantaneous tropical year. This was in conformity with the scale adopted by the in 1952.Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac (prepared jointly by the Nautical Almanac Offices of the United Kingdom and the United States of America, HMSO, London, 1961), at Sect. 1C, p.9), stating that at a conference "in March 1950 to discuss the fundamental constants of astronomy ... the recommendations with the most far-reaching consequences were those that defined ephemeris time and brought the lunar ephemeris into accordance with the solar ephemeris in terms of ephemeris time. These recommendations were addressed to the and were formally adopted by Commission 4 and the General Assembly of the Union in Rome in September 1952." This definition brings the observed positions of the celestial bodies into accord with Newtonian dynamical theories of their motion. Specifically, those tables used for most of the 20th century were (used from 1900 through 1983) and (used from 1923 through 1983). Thus, the 1960 SI definition abandoned any explicit relationship between the scientific second and the length of a , as most people understand the term. With the development of the in the early 1960s, it was decided to use as the basis of the definition of the second, rather than the revolution of the Earth around the Sun. Following several years of work, from the (Teddington, England) and from the (USNO) determined the relationship between the hyperfine transition frequency of the atom and the ephemeris second. |volume=1 |issue= 3|pages=105–107 |doi=10.1103/PhysRevLett.1.105 |bibcode=1958PhRvL...1..105M }} Using a common-view measurement method based on the received signals from , |volume=42 |issue=3 |pages=S10–S19 |doi=10.1088/0026-1394/42/3/S03 |bibcode = 2005Metro..42S..10L }} they determined the orbital motion of the about the Earth, from which the apparent motion of the Sun could be inferred, in terms of time as measured by an atomic clock. They found that the second of ephemeris time (ET) had the duration of 9,192,631,770 ± 20 cycles of the chosen caesium frequency. As a result, in 1967 the Thirteenth General Conference on Weights and Measures defined the second of atomic time as: This SI second, referred to atomic time, was later verified to be in agreement, within 1 part in 1010, with the second of as determined from lunar observations. (Nevertheless, this SI second was already, when adopted, a little shorter than the then-current value of the second of mean solar time. |volume=136 |issue= 5|pages=1906–1908 |doi=10.1088/0004-6256/136/5/1906 |quote= ... the SI second is equivalent to an older measure of the second of UT1, which was too small to start with and further, as the duration of the UT1 second increases, the discrepancy widens. |bibcode=2008AJ....136.1906M }}In the late 1950s, the caesium standard was used to measure both the current mean length of the second of mean solar time (UT2) ( ) and also the second of ephemeris time (ET) ( ), see |volume=4 |year=1968 |pages=161–165 |doi=10.1088/0026-1394/4/4/003 |issue=4 |bibcode=1968Metro...4..161E }}. As noted in page 162, the figure was chosen for the SI second. L Essen in the same 1968 article stated that this value "seemed reasonable in view of the variations in UT2".) During the 1970s it was realized that caused the second produced by each atomic clock to differ depending on its . A uniform second was produced by correcting the output of each atomic clock to (the rotating ), lengthening the second by about 1 . This correction was applied at the beginning of 1977 and formalized in 1980. In relativistic terms, the SI second is defined as the on the rotating geoid.See page 515 in |volume=38 |issue= 6|pages=509–529 |doi=10.1088/0026-1394/38/6/6 |last2=McCarthy |first2=D D |last3=Malys |first3=S |last4=Levine |first4=J |last5=Guinot |first5=B |last6=Fliegel |first6=H F |last7=Beard |first7=R L |last8=Bartholomew |first8=T R |bibcode=2001Metro..38..509N }} The definition of the second was later refined at the 1997 meeting of the to include the statement The revised definition seems to imply that the ideal atomic clock contains a single caesium atom at rest emitting a single frequency. In practice, however, the definition means that high-precision realizations of the second should compensate for the effects of the ambient temperature ( ) within which atomic clocks operate, and extrapolate accordingly to the value of the second at a temperature of . Possible future enhancements Today, the atomic clock operating in the microwave region is challenged by atomic clocks operating in the optical region. To quote Ludlow et al., |volume=96 |issue= 3|pages=033003 |doi=10.1103/PhysRevLett.96.033003 |id= |arxiv=physics/0508041 |bibcode=2006PhRvL..96c3003L }} “In recent years, optical atomic clocks have become increasingly competitive in performance with their microwave counterparts. The overall accuracy of single trapped ion based optical standards closely approaches that of the state-of-the-art standards. Large ensembles of ultracold alkaline earth atoms have provided impressive clock stability for short averaging times, surpassing that of single-ion based systems. So far, interrogation of neutral atom based optical standards has been carried out primarily in free space, unavoidably including atomic motional effects that typically limit the overall system accuracy. An alternative approach is to explore the ultranarrow optical transitions of atoms held in an optical lattice. The atoms are tightly localized so that Doppler and photon-recoil related effects on the transition frequency are eliminated.” The NRC attaches a "relative uncertainty" of 2.5 (limited by day-to-day and device-to-device reproducibility) to their based upon the 127I2 molecule, and is advocating use of an 88Sr ion trap instead (relative uncertainty due to linewidth of 2.2 ). See and |title=Trapped ion optical frequency standards }} Such uncertainties rival that of the NIST F-1 caesium atomic clock in the microwave region, estimated as a few parts in 1016 averaged over a day. |volume=42 |issue= 3|pages=S64–S79 |title=Atomic fountain clocks |doi=10.1088/0026-1394/42/3/S08 |bibcode = 2005Metro..42S..64W }} |accessdate=2009-08-19 }} SI multiples prefixes are commonly used to measure time less than a second, but rarely for multiples of a second (which is known as ). Instead, the non-SI units s, s, s, s, Julian centuries, and Julian millennia are used. Other current definitions For specialized purposes, a second may be used as a unit of time in time scales where the precise length differs slightly from the SI definition. One such time scale is UT1, a form of . McCarthy and Seidelmann refrain from stating that the SI second is the legal standard for timekeeping throughout the world, saying only that "over the years UTC ticks SI seconds has become either the basis for legal time of many countries, or accepted as the de facto basis for standard civil time". See also * * * * * * * * References External links * [http://www.npl.co.uk/server.php?show=ConWebDoc.1086 National Physical Laboratory: Trapped ion optical frequency standards ] * [http://resource.npl.co.uk/docs/networks/time/meeting3/klein.pdf High-accuracy strontium ion optical clock; National Physical Laboratory (2005)] * [http://inms-ienm.nrc-cnrc.gc.ca/research/optical_frequency_projects_e.html#optical National Research Council of Canada: Optical frequency standard based on a single trapped ion] * [http://physics.nist.gov/cuu/Units/second.html NIST: Definition of the second; notice the cesium atom must be in its ground state at 0 K] * Official BIPM definition of the second * Seconds and leap seconds by the USNO * The leap second: its history and possible future * [http://inms-ienm.nrc-cnrc.gc.ca/faq_time_e.html#10 What is a Cesium atom clock?] Category:SI base units Category:Units of time Category:SI base units Category:Units of time